The present invention relates to a heat developing apparatus and a heat developing method for developing and visualizing a latent image which has been formed on a heat-developing photosensitive film sheet.
The following Patent Document 1 discloses a heat-developing photosensitive material recording device in which a film, being an exposed recording material, is conveyed into a heat-developing section and developed as it is in contact with a heating drum and thereby heated. In this case, since the size of film varies depending upon the photographed object (photographed portion), the type of film conveyed to the heat-developing section also varies from time to time. Since the film is developed as it passes through a heating unit, accordingly, if the heating unit employs a heating drum, for example, the temperature of the area that is utilized for development (the area that is actually in contact with the film, which is called the “developing area” below) becomes lower than in other areas because the film removes heat from that contact area.
If the same area on the heating drum surface is always utilized for development, the temperature of the developing area is almost stable even when a plurality of recording films are developed in series, and therefore stable development becomes possible. When the film size is changed, however, the position, dimensions and shape of the developing area are different from the previous development operation. Consequently, problems tend to arise in that the temperature distribution in the new developing area does not become uniform immediately after changing to a different film size, and uneven development is easily caused.
This problem is remarkably found particularly when the recording material is changed from a small size to a large size. Since high image quality is required in the medical field, high image-quality recording film is employed. But, because the effect of heat on high image-quality recording film like the above during development is very much, the above-mentioned uneven development tends to occur.
The following Patent Document 2 discloses an image forming apparatus in which, in order to prevent overheating of the sheet non-passage area on a fixing roller when recording material is continuously fed into the fixing unit, the feeding interval of the recording material is changed during the continuous feed between fixing at the first set temperature and fixing at the second set temperature. In a fixing unit like the above, however, temperature non-uniformity is caused on the fixing roller surface, because, although the surface temperature of the recording material passage area on the fixing roller becomes lower while the recording material passes through it, heat is hard to be removed from the recording material non-passage area of the fixing roller. This temperature non-uniformity is remarkably evident when the recording material continuously passes through the fixing roller surface. If the temperature is set high enough for the sheet passage area in this operation, the sheet non-passage area becomes excessively hot. This tendency is particularly marked when the set temperature of the fixing roller is changed, when the size of recording material is changed, and for a while after the fixing roller surface reaches the temperature for image forming.
The following Patent Document 3 discloses a heat-developing apparatus, using a heat-developing method that can control image-quality degradation due to the temperature drop of heating members resulting from continuous processing, and also continuously heat-develops the heat-developing sheets of different sizes on which an exposed latent image has been formed, to reduce the continuous process time, in which the minimum required temperature restoration time for heat-developing the following heat-developing sheet is determined from the physical data of the heat-developing sheet currently being developed, and the development of the following heat-developing sheet to be developed next is started after the minimum temperature restoration time has elapsed.
The heat-developing apparatus of Patent Document 3 employs the same method as for Patent Document 2, the temperature of which is controlled by a single sensor in the lateral direction and the apparatus carries out nothing but waiting until the temperature distribution in the lateral direction becomes uniform. Processing capacity cannot improve when the size of the recording material or heat-developing sheet is changed frequently.
The following Patent Document 4 discloses a fixing device provided on a copying machine, printer, facsimile machine, or the like. A fixing device of this type is equipped with a heating member, where the surface temperature of fixing roller is sensed by a thermal sensor and the surface temperature of the fixing roller is controlled via signals to maintain at a specified temperature by varying the heat from the heating member. That is to say, it is in an ON/OFF system, by which power to the heating member is turned ON if the surface temperature of the fixing roller is lower than the specified temperature and turned OFF if higher, or electrical power to the heating member is controlled accordingly. This temperature control is achieved using signals from a thermal sensor such as thermistor installed in contact with or close to the fixing roller surface, where the surface temperature of the fixing roller is sensed at one location.
However, it frequently happens that the surface temperature of the fixing roller is not at a constant temperature particularly across its whole width in the axial direction resulting from airflow inside or outside the apparatus, operating conditions, sheet size, or inherent differences among machines. Consequently, the surface temperature of the fixing roller near the portion where the thermal sensor is installed is controlled to the specified temperature but the specified temperature may not be maintained at portions away from the thermal sensor. Under this condition, problems arise in that fixing characteristics do not become uniform so that stable and favorable fixing cannot be achieved.
In Patent Document 4, in order to maintain nearly constant temperature across the whole width of the fixing roller in the fixing device, even when the temperature condition of the fixing roller is different in the axial direction, the fixing roller is divided into two heating areas, nearly equally divided into right and left portions in the axial direction. A high-temperature heating member of each heater is provided across the whole heating area, a thermal sensor for sensing the surface temperature of each heating area is provided, and temperature balance on the fixing roller surface is controlled so that each heating area is maintained at the specified temperature.
The following Patent Document 5 discloses an apparatus in which a film sheet is subjected to heating and conveyed while it is wound around a heating drum and pressed by opposed rollers. This apparatus is capable of processing three different sized sheets of film of 14×17 inch, 14×14 inch and 11×14 inch having the same width, by the same heater pattern. When processing of 10×12 inch or 8×10 inch is also desired, however, the apparatus requires a stand-by time until the drum is restored to a uniform temperature due to the changed size. This stand-by time can become much longer when the size is changed after continuous processing of film sheets of the same size because the temperature difference between the film-passage portion and non-passage portion becomes much greater. The stand-by time also varies depending upon the type of film and temperature setting for heat-development. Accordingly, the processing capacity per unit time is tremendously low.    [Patent Document 1] Tokkai Hei No. 11-65070    [Patent Document 2] Tokkai Hei No. 05-6043    [Patent Document 3] Tokkai No. 2002-244266    [Patent Document 4] Tokkai Hei No. 05-53463    [Patent Document 5] Tokuhyou Hei No. 10-500497